This invention relates to a mobile communication system which includes a base station and a mobile station communicable with the base station and, in particular, to a transmitter and a receiver used in the mobile communication system.
In general., such a mobile communication system of the type described is known in the art as a cellular system. Conventionally, a wide variety of the cellular systems have been proposed, which may be, such as a standard system in Japan abbreviated to PDC:RCR STD 27A, a North American standard system abbreviated to TIA IS54, an Europe Standard system abbreviated to ETSI GSM, and another North American standard system abbreviated to TIA IS95. Herein, it is to be noted that the PDC:RCR STD 27A uses a time division multiple access (TDMA) method while the TIA IS95 uses a code division multiple access (CDMA) method.
In the TDMA method, it is difficult to avoid a frequency interference which might result from use of an identical frequency. Therefore, the identical frequency can not be assigned to adjacent ones of cells but should be used in remote cells. Accordingly, the TDMA method has a shortcoming that a utilization efficiency of frequencies is low.
On the other hand, the CDMA method is advantageous in that the same frequencies can be used in every one of cells and that a path diversity effect can be accomplished by the use of a simple rake receiver which is well known in the art.
However, it is difficult in the CDMA method that an interference among channels can not be completely rendered into zero. From this fact, it is readily understood that the CDMA method is not suitable for simultaneously transmitting a plurality of information signals which are individually produced from different signal sources.
Herein, proposal has been made in U.S. Pat. No. 4,799,252 as regards another mobile communication system that comprises a plurality of base stations and a plurality of mobile stations communicable with the base stations through forward links and reverse links and that carries out communication by using a combination of time division multiplexing (TDM) and code division multiplexing (CDM). The forward links are directed from the base stations towards the mobile stations while the reverse links are directed from the mobile stations towards the base stations.
Herein, it is to be noted in the above-referenced U.S. Patent that the combination of the TDM and the CDM is only used in the forward links from each of the base stations to the mobile stations to accurately synchronize information signals of all channels that are sent from each base station to the mobile stations. This shows that consideration is made only about a connection from a single base station to the mobile stations, N in number, where N is an integer greater than unity. In other words, no consideration is made at all in the U.S. Patent as regards multiple connections from the base stations (N in number) to the mobile stations (N in number) or from the base stations (N in number) to a single one of the mobile stations.
From this fact, it is readily understood that, when the TDM and the CDM methods are used for individually transmitting a plurality of information signals, synchronization should be accurately established using the individual information signals in the U.S. Patent.
Alternatively, a communication method is disclosed in Japanese Unexamined Patent Publication No. Hei 4-351130, namely, 351130/1992 so as to transmit and receive a data signal sequence by the CDM method. More specifically, a plurality of transmission data signals are subjected to the code division multiplexing by the use of the same spread codes by a transmitter and successively transmitted at time intervals displaced relative to each other. Thereafter, a receiver receives the plurality of the transmission data signals by the use of an inverse filter. In this method, the inverse filter is controlled so that reception response of the inverse filter approximates a delta function of Kvonecker. However, this method is disadvantageous in that a multiplex number for the CDM is restricted because a time deviation of the transmission data signals must be longer than a sum of a chip time of the spread code and a delay expansion in a transmission path.